Implantable medical devices are available for treating cardiac arrhythmias by delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation. Single chamber devices are available for treating either atrial arrhythmias or ventricular arrhythmias, and dual chamber devices are available for treating both atrial and ventricular arrhythmias. Rate zone classifications typically include normal sinus rhythm, tachycardia, and fibrillation.
Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to the absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” In modern implantable cardioverter defibrillators, the physician programs the particular therapies into the device ahead of time, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber, in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher energy cardioversion pulse may be selected. Therapies for tachycardia termination may also vary with the rate of the detected tachycardia, with the therapies increasing in aggressiveness as the rate of the detected tachycardia increases. For example, fewer attempts at anti-tachycardia pacing may be undertaken prior to delivery of cardioversion pulses if the rate of the detected tachycardia is above a preset threshold. For an overview of tachycardia detection and treatment therapies reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al.
Ventricular tachycardia (VT) may be debilitating, but is not necessarily an immediately life-threatening situation. Cardiac output tends to be compromised due to the disorganized contraction of the myocardial tissue resulting in a patient feeling weak, dizzy or even fainting. Ventricular tachycardia may, however, degenerate into a more unstable heart rhythm, leading to ventricular fibrillation. Therefore in most cases, it is desirable to immediately treat a detected VT, either with anti-tachycardia pacing therapies or cardioversion shocks. Because VT can often be terminated by known anti-tachycardia pacing therapies, these therapies are generally delivered first, because they are less painful to the patient, then followed by high-energy shock therapy if necessary.
However, in some cases, a patient may be diagnosed with a recurrent slow-rate ventricular tachycardia that is not associated with symptoms of hemodynamic compromise. When a recurrent VT is repeatedly detected by an ICD device, the patient will normally undergo a preset menu of tiered therapies, which may conclude with shock delivery in order to terminate the VT. Therefore, a patient having a recurrent VT may be repeatedly subjected to painful shock therapies. In a patient having recurrent, but hemodynamically stable, slow VT, such repeated shock therapy may be undesirable since the condition is not immediately life-threatening and not expected to deteriorate into a more serious tachycardia. An implantable cardioverter defibrillator device capable of delaying or suspending a high-energy shock therapy in response to detecting a stable, low-rate ventricular tachycardia is therefore needed.